Figure 16-2. Counter Unit Block Diagram
TOVn
(Int.Req.)
DATA BUS
TOSC1
TOSC2
count
clear
T/C
Oscillator
clk Tn
TCNTn
Control Logic
Prescaler
direction
clk
bottom
top
I/O
Signal description (internal signals):
count
direction
clear
Increment or decrement TCNT2 by 1.
Selects between increment and decrement.
Clear TCNT2 (set all bits to zero).
Timer/Counter clock.
clkT2
top
Signalizes that TCNT2 has reached maximum value.
Signalizes that TCNT2 has reached minimum value (zero).
bottom
Depending on the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clkT2). clkT2 can be generated from an external or internal clock source,
selected by the Clock Select bits (CS22:0). When no clock source is selected (CS22:0 = 0) the
timer is stopped. However, the TCNT2 value can be accessed by the CPU, regardless of
whether clkT2 is present or not. A CPU write overrides (has priority over) all counter clear or
count operations.
The counting sequence is determined by the setting of the WGM21 and WGM20 bits located in
the Timer/Counter Control Register (TCCR2A). There are close connections between how the
counter behaves (counts) and how waveforms are generated on the Output Compare output
OC2A. For more details about advanced counting sequences and waveform generation, see
”Modes of Operation” on page 143.
The Timer/Counter Overflow Flag (TOV2) is set according to the mode of operation selected by
the WGM21:0 bits. TOV2 can be used for generating a CPU interrupt.
16.4 Output Compare Unit
The 8-bit comparator continuously compares TCNT2 with the Output Compare Register
(OCR2A). Whenever TCNT2 equals OCR2A, the comparator signals a match. A match will set
the Output Compare Flag (OCF2A) at the next timer clock cycle. If enabled (OCIE2A = 1), the
Output Compare Flag generates an Output Compare interrupt. The OCF2A Flag is automatically
cleared when the interrupt is executed. Alternatively, the OCF2A Flag can be cleared by soft-
ware by writing a logical one to its I/O bit location. The Waveform Generator uses the match
signal to generate an output according to operating mode set by the WGM21:0 bits and Com-
pare Output mode (COM2A1:0) bits. The max and bottom signals are used by the Waveform
Generator for handling the special cases of the extreme values in some modes of operation
(”Modes of Operation” on page 143).
Figure 16-3 shows a block diagram of the Output Compare unit.
140
ATmega169P
8018A–AVR–03/06
ATMEL [ ATMEL ]
